Ion-selective electrode

ABSTRACT

An electrode for analyzing the concentration of a specific ion in a solution contains an electroactive plate consisting of a salt in which one ion is a radical-ion. Said electroactive plate is selective to the specific ion.

1 Dec. 17, 1974 United States Patent [1 1 Sharp et a1.

Ross et 204/195 M Kedem et a1... 204/195 L 204/195 L 204/195 M 204/195 M W l M w "re "0 Suc S ur. RPF

[ ION-SELECTIVE ELECTRODE [22] Flled: June 1972 Primary ExaminerT. Tung Attorney, Agent, or Firm-Toren, McGeady and Stanger [21] Appl. No.: 264,390

[30] Foreign Application Priority Data June 17, 1971 Sweden......,................,....... 7856/71 ABSTRACT consisting of a salt in which one ion is a radical-ion. Said electroactive plate is selective to the specific ion.

[56] References Cited UNITED STATES PATENTS 18 Claims, 6 Drawing Figures 2,930,967 3/1960 Laird et al. 204/195 R PATENTED DEC] 7 I974 -IOO I sum 2 or 3 Fig. 3

Fig.4

M.Cu

MPb

PATENTEUIJEBI 71974 855,097

SHEET 3 BF 3 Fig. 5 P

' IOO ION-SELECTIVE ELECTRODE The invention is concerned with an ion-selective electrode for measuring the concentration of ions in a solution.

The activities of ions in solution can be determined with an electrochemical cell. This well-established technique exploits electrodes of the type where the potential of this electrochemical half-cell is determined by the activity of chloride ion in the solution. A complete cell'must necessarily consist of two such half-cells. As a further example the glass electrode employed for pl-l-measurements may be cited. The glass forms a membrane which is supplied with electrical contacts as follows The complete half-cell (2) constitutes the glass electrode, or, when the membrane consists of a different material, represents the general construction of many ion-selective electrodes. A particular example of an ion-selective electrode may be written Ag,AgCl/K Cl, FlLaF membrane/F The electrode potential changes when the fluoride ion activity in the test solution is altered (British Pat. No. 1,131,574).

The half-cell (1) may be placed in a special chamber which contains a solution of known constant chloride ion activity. Electrical contact to the remainder of the cell is made through a liquid-junction. Such an arrangement effectively maintains the potential of the half-cell constant, and this can be used as a reference point for further potential measurements. This assembly is known as a reference electrode.

The activity of fluoride ion in a given solution can be determined with the aid of the ion-selective electrode (3). If the potential difference between the two halfcells is measured with a voltmeter, the following relation, called Nernsts law, is obtained E E, RT/nF In [F'] where E represents the measured potential, E is constant, R the gas-constant, T the absolute temperature, n the valence of the measured ion, F the Faraday and [F'] the fluoride ion activity or concentration. At 25C, expression (4) may be simplied to E= E, 59.2 log [F'] where E and [5,, are expressed in mV. In practice RT/nF is usually replaced by the slope of the experimentally determined calibration line.

Many half-cells are affected by more than one species of ion. For measurements of a specified ion the adverse effects of the presence of other ions in the test solution is termed interference. Interference by an ion of typej upon an electrode supposedly selective to reference ion, i, may be represented by a mixed response equation.

where m is the valence of ion, i, with activity 11,, and n the valence of ion,j, with activity a,.

Values of K greater than unity indicate that interference by ion,j, is considerable and measurements of the activity, a,, are thus complicated by the presence of significant concentrations of ion j. i

A large number of membrane materials have been examined during recent years. These have included different glasses for hydrogen-, sodium-, potassiumand ammonium-ion electrodes, and inorganic salts for fluoride, chloride, bromide, iodide, cyanide, sulphide, copper, lead, cadmium and silver ion electrodes. Liquid state membranes have been employed for potassium, calcium, nitrate and perchlorate ions. Extensive efforts are being made on a worldwide scale to discover new materials which provide electrodes for additional ions and which provide electrodes with improved behaviour. Materials which may be considered must exhibit low solubility in the intended measuring medium, possess appreciable electrical conductivity, and be capable of establishing a rapid electrochemical equilibrium with a given ion in the test solution.

When an electron is added to a neutral molecule the molecule becomes negatively charged, and since the electron is not spin-paired the resulting'species possesses radical character. In such a case a radical-anion results. Similarly removal of an electron will yield a radical-cation. In the case of suitable organic molecules those electrons which are added or removed enter or leave the rr-electronic system of the molecule, and the resulting distribution of charge stabilizes the radical species. Orbital overlap between such radical-ions in the condensed phase generally leads to marked electrical conductivity. In particular, the complexes or salts formed by radical-ions and suitable counterions are good electronic conductors. During recent years many radical-ions have been synthesised which are reasonably stable even in contact with water. Such species may generally be prepared by straight forward chemical or electrochemical oxidation or reduction of the parent neutral molecule. These reactions are normally performed in organic solvents such as acetonitrile, methylene chloride or methanol.

The electrode of the invention comprises an ionselective electrode for the analysis of the concentration of inorganic, organic and metal-organic ions in a solution, comprising an electrically insulating electrode holder, an electroactive plate in said electrode holder, and an electric conductor connected to said electroactive plate, and is characterized in that the electroactive plate consists of a salt in which one ion is a radical-ion. The countepion is preferably the ion to be determined in a test solution. The counter-ion may be an inorganic, organic, or metal-organic ion.

Consequently, the essence of the invention is the use of radical-ion salts as electroactive substances in ionselective electrodes.

For example, it has been shown that 9- dicyanomethylene-2,4,7-trinitrofluorene (DTF) may be dissolved in acetonitrile and treated with lithium io- I (IN dide whereby the corresponding radical-anion is obtained in the form of its lithium salt. Other cation salts of DTF may be obtained by suitable precipitation (IN methods. A wide range of materials have been converted to corresponding radical-ion salts and these latter found This compound (IV) has a selectivity which is approxisuitable for use as electrode sensing materials. As exmately equal to that of compound (Ill). amples of the starting organic substances may be men- Another useful structure is tioned structures of the type NO ON Y X1 X2 X8)\X1 X10 X3 X7 X2 1; X9 X4 X6 5 X X3 X8 X7 X5 X4 I where X H, N0 Cl, Br, I, F, C -H CH ,l C H or higher aliphatic hydrocarbon chain, CN, OCl-l where X and X must be 0 or OC l-l OC H or other alkoxy-grouping. The substituv ents offer a wide variety of combinations. if, for exam- 0N ple, x,=x,,=x,,=x,=ri; X,= ,=X -r-1o the resulting product will be 9-dicyanomethylene-2,4,5,7-tetranitrofluorene, often abbreviated DTTF. lf also X =H the resulting product will be DTF, referred to above.

Modifications of the basic structure (I) are possible, the other Xs representing the groupings specified for for example: the compound (I). All the above mentioned structures H g V A form radical-anions. X8 X1 Those structures listed below form radical-cations.

X6 X7 X8 X1 X6 X3 X5 X4 H X5/ X4 X3 \X2 v1 where X represents the groupings specified for the compound (1). This compound (ll) is ion-selective for where X may be H, CH3. CH C -,H, or higher hydrosilver, copper. nickel, and other ions. carbon chain, OCH OC H or other alkoxy-grouping,

Different structures may also be used: C H Cl, Br, I, F. Preferably X,=X =X =X 4' CN )2 X6 l X8 X9 X10 X11 X12 /X1 1 CN \N -N\ NC X1 X7 X6 X5 X4 3 X2 v11 L X3 X2 N 111 where X represents the groupings specified for the where X represents the groupings specified for the compound (VI) and where X,=X =X =X,,. The radical compound (1). if X=l-l the resulting compound will be cations (VI) and (Vll) form compounds useful for elecll,l l,l2.12-tetracyanonaphto-Z,o-kinodimethane, trodes for measuring perchlorate, tetrafluoroborate, often abbreviated TNAP, The compound (III) is ionnitrate, and other ions.

selective for certain organic ions, such as tetra- A modification of the structure (Vll) is of interest:

phenylarsonium ion-, tetrabutylammonium ion, and also for certain inorganic ions such as copper, lead,

mckel- X6 X7 X8 X9 Another useful structure is:

X5 -NH2 (IN X4 3 X2 X1 vm X6 X7 xx 1 X CN u X1 where X -,=H, Cl, Br, I, F, CH C H or other aliphatic X4 X3 X0 hydrocarbon chain and other Xs represent the group- J; IV ings specified for the compound (Vl). Materials of this type may give sulphate ion selective electrodes, espewhere X represents groupings specified for compound cially for X =Cl. (I). In addition X and X, may be O, or Substituted thianthrenes may also be used:

where X, S, N. C or O and other X's represent groupings specified for the compound (Vl). Even some aliphatic structures may be used:

where X represents the groupings specified for the compound (VI).

Tetrathiotetracenes, (Xl) or substituted tetrathiotetracenes may be converted to radical-cation species which form sparingly soluble salts with several anions.

Benzothiazolone azine. (Xll) and similar compounds form radical-cations which yield salts suitable for electrodes showing high selectivity towards perchlorate. perrhenate. tetrafluoroborate and iodide ions. R is an alkyl group.

I i N/ KN i. l a.

The compounds I X are useful for producing salts with inorganic ions, The compounds I IV are useful for producing salts with organic metal-organic cations, and the compounds V] X are useful for forming salts with organic and metal-organic anions.

A mixture of two or more radical-ion salts is often to be recommended, because the electrode becomes more stable and less sensitive to light.

Electroactive materials may be prepared in general from substances of types I Xll by suitable oxidation or reduction and precipitation methods. If the radical is an anionic radical. it is preferred first to prepare an easily soluble salt of the radical. for example the sodium or lithium salt. Said salt is now reacted with. for example. Agfl Cu. Ni, Co. Pb, Zn, Cd'. Hg2+ Hgz'li 112+ Ti-H" v4+, VO2+ Alibi S -i-i S 2+ Ca NHJ' to form a salt having a low solubility. This salt, after purification and drying, may be pressed to form electroactive plates at an adequate pressure, usually l.000-l0,000 kg/cm If the radical is a cationic radical it is preferred first to prepare an easily soluble salt, such as the acetate, and to react said easily soluble salt with, for example C10 SO PO HPOE". l-l POf, ReOf, I0 BrOf, MnOf, SCN', N0 7, S 0 CN, 8*, Fe(CN) FE(CN).;", to form a salt having a low solubility, which is now purified, dried, and pressed to form an electroactive plate. Other useful Counter-ions are (C6H5)4B+ (CH3)4N+, (C2H5)4N+, 3 7)4 v -I Q)'1 Q e 5)4 6 5)4 a (C H Sb (CH )Hg (C H )Hg methoxyethyl mercury ion, ethoxyethyl mercuryv ion, anions and cations of amino acids, and ions of a quaternary ammonium compound having the structure in which R is hydrogen, an alkyl or an aryl group, at least one substituent being an alkyl or aryl group. The electroactive plate obtained can be mounted in various ways in the electrode holder. Sealing between the electroactive plate and the insulating electrode holder (which may be of glass, PVC, polystyrene etc.) may be effected by in situ pressing or by the use of an appropriate adhesive, e.g., epoxy-resin or silicone rubber.

An alternative method of mounting the electroactive plate is to put it in contact directly. or through some conducting material such as graphite paste, with a metal base which is fitted with a contact. All conductive surfaces except that of the electroactive plate are insulated, e.g., with plastic-enamel or silicone rubber. The electronic conductivity exhibited by radical-ion salts allows such dry contact constructions to be fully exploited.

A further method involves dissolution of the radicalion salt in an otherwise inert plastic matrix in such a quantity that the resulting composition possesses suitable electrical resistance. For example. the copper salt of DTF may be dissolved in polyacrylonitrile in a quantity of 5 50 percent by weight, for example, and the dried film may be mounted as a membrane or electroactive plate as described above.

The electro-active material may also be precipitated directly upon a metallic conductor. Radical-ion salts may be formed upon the surface ofa conductor during electrolysis. Perchlorate sensitive plates of the compound Vlll have been made in this way with conductors of platinum or graphite.

If the radical-ion salt is dissolved in a solvent which is immiscible with water, liquid membrane constructions may be applicable. The liquid may be absorbed by a sintered glass or ceramic disc which provides the mechanical support for the membrane or plate.

ldeally, single crystal membranes are expected to provide the most advantageous conditions for electrode functioning. However, economic considerations favour the adoption of several of the different methods described above. The electrode. used with radical-ion salts results in electrodes with electroactive plates 0. l-2 mm thick and total electrical resistances of a few hundred ohms.

The invention will now be described with reference to the drawings. HO. 1 shows an apparatus containing an electrode according to the invention. FIG. 2 shows another embodiment of the elctrode. FIGS. 3-6 illustrate the results of measurements with various electrodes according to the invention.

The apparatus of FIG. 1 comprises a vessel 8 for a test solution 5, a measuring electrode 6, a reference electrode 7, and a voltmeter 10. The measuring electrode comprises a tubular electrode holder 6 of glass. The lower end of the tube 6 is closed by means of an electroactive plate 1 consisting of the copper salt of DTF. The tube 6 is partially filled with a solution of CuCl A silver wire 3 having a surface coating of AgCl extends down into the solution 2. The upper end of the silver wire is connected to the voltmeter 10. The reference electrode 7, known per se, has a similar structure, but its bottom opening is closed by means of a plug 9 of, for example, asbestoes fibres or a glass filter. The silver wire 4 is connected to the voltmeter 10. In operation, the electrode potential changes with the activity or concentration of Cu in the test solution 5 according to the formula (4) in which n =+2. In the electrode of FIG. 2 an electroactive plate 21 has been fastened to a metal base 23 by means of a graphite layer 22. A metal wire 24 is fastened to the metal base 23. The assembly is covered with a layer 20 of plastic-enamel, the layer 20a covering the main part of the metal wire 24, the layer 20b covering the metal plate 23, the graphite layer 22 and the electro-active plate 21 except for its external main surface.

Two lead ion selective electrodes are compared in H0. 3. The upper-curve 31 represents the lead salt of TNAP, and the lower curve 32 represents the lead salt of DTF. Both electrodes are useful as lead ion activity indicators over a wide range, IO M Pb. Li, Na K. Ca N ions do not interfere. Cu, Ag* and Hg ions interfere strongly and should be present in low concentrations during lead ion activity determinations.

Three copper ion selective electrodes are compared in H6. 4. The upper curve 41 represents the copper salt of TNAP, the middle curve 42 represents the copper salt of DTTF, and the lower curve 43 represents the copper salt of DTF. Ag* and HG ions interfere but PB and Ni may be present in significant concentrations without causing interference. Alkali-metal ions cause no interference.

Two tetraphenylarsonium ion selective electrodes are shown in FIG. 5. The upper curve 51 represents the tetraphenylarsonium salt tetraphenylarsonium DTTF, and the lower curve 52 represents the tetraphenylarsonium salt of DTF. These electrodes have been used to titrate perchlorate ion with tetraphenylarsonium chloride ion potentiometrically. The slope of the line in FIG. 5 should have been 59.2 mV per decade at 25C. It has been found that the purity of the material influences the slope greatly. the purer the material the more nearly the slope approaches ideality. The preparative method is also of importance. Poor electrode behaviour can arise from cracks or pits which are formed in the electroactive plate. Optimum conditions appear to involve pressing at 700 kg/cm and 120C under vacuum with teflon-coated. diamond-polished die faces.

H0. 6 refers to a perchlorate ion selective electrode prepared by electrolysis (oxidation in methylene chloride) upon a platinum base of a solution of o-tolidine (structure Vll with X,=X =X =X,,=H and X =X =CH and X,=X,,=X,,=X ,=X, =X =H) in the presence of tetraethylammonium perchlorate. The platinum base was previously cleaned electrolytically. Tolidine perchlorate was formed directly upon the metal. Other ions, e.g., halide, show low interference during perchlorate ion activity determination. Alternatively, otolidine may be oxidised chemically in the presence of perchlorate ions, whereupon the desired electroactive material is obtained as a precipitate.

These and other measurements show that ion selective electrodes for cations may be prepared from radical-ion salts derived from structures l-V. Cations may be chosen widely and may include organic and organometallic types. Electrodes selective to anions may similarly be derived from suitable radicalcation salts.

Low values K in equation (6) reflect only slight interference by other ions. Values of K for an electrode made from benzothiazolone azine perchlorate, Xll, were found to be for CI 8.5 10', N0 1.1 lO, Br 7.4 10 F 5.1 l0", Ac 4.6 10 OH 8.3 10*, C10,, 9.1 10 These values show that the ions mentioned can be present in concentrations 1,000 times greater than those of perchlorate ion without causing appreciable interference. The perchlorate ion activity range was 0.1 M 10* M. Iodide should not be present in concentrations greater than one-hundredth that of perchlorate ion.

The invention will now be described by several examples, it being understood, however, that these examples are given by way of illustration and not by way of limitation and that many changes may be effected without affecting in any way the scope and spirit of this invention as recited in the appended claims.

The preparation of ion radical electrode materials can be made as follows.

0.92 g 2,4,7-trinitro-9-fluorenone was suspended in ml boiling methanol and 2 drops of piperidine were added as catalyst. 0.58 g dimalonitrile was then added. The solution was boiled for 10 minutes and allowed to cool. Yellow crystals were precipitated and they were filtered off, washed with methanol and dried. They were recrystallized from acetonirtile and the resulting product was washed with acetonitrile and large quantities of anhydrous ether. The yield was 380 mg 9- dicyano-methylene-2,4,7-trinitrofluorene. The ion radical can be prepared by dissolving 380 mg 9-dicyanomethylene-2,4,7-trinitrofluorene in acetonitrile and adding 200 mg lithium iodide dissolved in acetonitrile. The solution is boiled for ID minutes, and is allowed to cool. A black precipitate forms and it is filtered off, washed with a small quantity of acetonitrile and large quantities of anhydrous ether, dried and weighed. The yield was 350 mg lithium salt of the 9- dicyanomethylene-Z,4,7-trinitrofluorene ion radical.

In order to prepare copper-9-dicyanomethylene- 2,4,7-trinitrofluorene 0.21 g of the lithium salt was dissolved in methanol. 69 mg copper(ll)-nitrate was also dissolved in methanol and the cool solutions were mixed. A fine precipitate forms immediately and it is filtered off and washed with methanol and large quantities of anhydrous ether. It is dried and used for making copper selective electrodes.

in order to prepare an electrode selective to tetraphenylarsonium ions a material can be prepared as follows. 2 g tetraphenylarsoniumchloride was dissolved in a small quantity of absolute ethanol and anhydrous ether was added to precipitate the tetraphenylarsonium chloride. The precipitate was filtered off, washed well with anhydrous ether and the procedure was repeated once more. The yield was then L67 g purified tetraphenylarsonium chloride.

l g purified tetraphenylarsonium chloride and 0.5 g potassium iodide were dissolved separately in distilled water and the resulting solutions were mixed. A white precipitate formed and it was filtered off, washed with large quantities of distilled water and dried. The precipitate was tetraphenylarsonium iodide.

200 mg lithium-9-dicyanomethylene-2,4,7- trinitrofluorene and 270 mg tetrapheriylarsonium iodide were dissolved separately in absolute methanol. When the solutions were mixed a fine black precipitate formed immediately. It was filtered off, washed with large quantities of methanol and anhydrous ether and dried. The yield was 240 mg tetraphenylarsonium-9- dicyanomethylene-Z,4,7-trinitrofluorene which can be used directly for making the electrodes.

What is claimed is:

1. An ion-selective electrode for the analysis of the concentration of inorganic, organic and metal-organic ions in a solution, comprising an electrically insulating electrode holder, an electroactive plate in said electrode holder, and an electric conductor connected to said electroactive plate, characterized in that the electroactive plate consists of a salt in which one ion is a radical-ion possessing an electron which is not spinpaired.

2. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure NC /CN in X=H, N02, Cl, Br, 1, F, C gH5, CH3, C2H5 or another aliphatic carbon chain, CN, OCH OC- H OC H or another alkoxy group.

3. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure X8 A X1 X7 X2 X3 X2 NC in X=H, N02, Cl, Bl, I, F, C3H5, CH3, C2H5 Or another aliphatic carbon chain, CN, OCH OC H OC H or another alkoxy group.

5. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure X4 X3 x2 NC in which X=H, N0 Cl, Br, I, F, C H CH C H or another aliphatic carbon chain, CN, OCH OC H OC H or another alkoxy group, and in which X;, and X may contain =0 or 6. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure in which X=H, N0 Cl, Br, I, F, C 11 CH C H or another aliphatic carbon chain, CN, OCH OC H OC H-, or another alkoxy group,.and in which X and X are =O or 7. Anelectrode as claimed in claim I wherein said one ion of the radical-ion salt has the structure X9 X10 X11 X12 X1 X6 X5 X2 VII in which X may be H, CH C H C,-,H or another aliphatic carbon chain, OCH;,, OC H or another alkoxy group, C.,H,,, Cl. Br, I, or F.

9. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure in which X .,=H, Cl, Br. I, F, CH C- H or another aliphatic carbon chain, and the other Xs may be H, CH C H C H or another aliphatic carbon chain, OCH OC H or another alkoxy group, C H Cl, Br, I, or F.

10. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure in which X is S, N, C or O, and the other X's may be H. CH C H C;,H,, or another aliphatic carbon chain, OCH;,. OC H or another alkoxy group, C H Cl, Br, I, or F.

ll. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure N N\' 125 W i:

in which X may be H. CH C H C H or another aliphatic carbon chain, OCH OC- H or another alkoxy group, C H Cl. Br, I, or F.

12. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure 13. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure in which R is an alkyl group.

14. An electrode as claimed in claim 1, wherein the electroactive plate is arranged as a membrane.

15. An electrode as claimed in claim 1, wherein the salt is supported by compact. solid material which is conductively connected to the electric conductor.

16. An electrode as claimed in claim 1, wherein the salt is supported by a plastic material.

17. An electrode as claimed in clalim 1, wherein the salt is dissolved in a solvent.

18. An electrode as claimed in claim 1, wherein the electroactive plate consists of a mixture of at least two radical-ion salts.

XII 

1. AN ION-SELECTIVE ELECTRODE FOR THE ANALYSIS OF THE CONCENTRATION OF INORGANIC, ORGANIC AND METAL-ORGANIC IONS IN A SOLUTION, COMPRISING AN ELECTRICALLY INSULATING ELECTRODE HOLDER AN ELECTROACTIVE PLATE IN SAID ELECTRODE HOLDER, AND AN ELECTRI CONDUCTOR CONNECTED TO SAID ELECTROACTIVE PLATE, CHARACTERIZED IN THAT THE ELECTROACTIVE PLATE CONSISTS OF A SALT IN WHICH ONE ION IS A RADICAL-ION POSSESSING AN ELECTRON WHICH IS NOT SPIN-PAIRED.
 2. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 3. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 4. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 5. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 6. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 7. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 8. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 9. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 10. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 11. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 12. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 13. An electrode as claimed in claim 1 wherein said one ion of the radical-ion salt has the structure
 14. An electrode as claimed in claim 1, wherein the electroactive plate is arranged as a membrane.
 15. An electrode as claimed in claim 1, wherein the salt is supported by compact, solid material which is conductively connected to the electric conductor.
 16. An electrode as claimed in claim 1, wherein the salt is supported by a plastic material.
 17. An electrode as claimed in clalim 1, wherein the salt is dissolved in a solvent.
 18. An electrode as claimed in claim 1, wherein the electroactive plate consists of a mixture of at least two radical-ion salts. 